U.S. patent application number 16/400010 was filed with the patent office on 2019-11-14 for compact millimeter wave system.
The applicant listed for this patent is TEXAS INSTRUMENTS INCORPORATED. Invention is credited to Argyrios Dellis, Adam Joseph Fruehling, Juan Alejandro Herbsommer.
Application Number | 20190346814 16/400010 |
Document ID | / |
Family ID | 68464709 |
Filed Date | 2019-11-14 |
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United States Patent
Application |
20190346814 |
Kind Code |
A1 |
Fruehling; Adam Joseph ; et
al. |
November 14, 2019 |
COMPACT MILLIMETER WAVE SYSTEM
Abstract
A millimeter wave apparatus, with a substrate, a transceiver in
a first fixed position relative to the substrate, and a gas cell in
a second fixed position relative to the substrate. The clock
apparatus also comprises at least four waveguides.
Inventors: |
Fruehling; Adam Joseph;
(Garland, TX) ; Herbsommer; Juan Alejandro;
(Allen, TX) ; Dellis; Argyrios; (Allen,
TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TEXAS INSTRUMENTS INCORPORATED |
Dallas |
TX |
US |
|
|
Family ID: |
68464709 |
Appl. No.: |
16/400010 |
Filed: |
April 30, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62669598 |
May 10, 2018 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01P 3/121 20130101;
G04F 5/14 20130101; H03L 7/26 20130101; G01S 7/35 20130101; H04B
1/40 20130101 |
International
Class: |
G04F 5/14 20060101
G04F005/14; H04B 1/40 20060101 H04B001/40; H03L 7/26 20060101
H03L007/26 |
Claims
1. A millimeter wave apparatus, comprising: a substrate; a
transceiver in a first fixed position relative to the substrate; a
gas cell in a second fixed position relative to the substrate; a
first waveguide affixed relative to the substrate, the first
waveguide having a first end coupled to the transceiver and a
portion, along a first dimension, having a second end proximate a
first portion of the gas cell; a second waveguide affixed relative
to the substrate, the second waveguide having a first end coupled
to the transceiver and a portion, along a second dimension, having
a second end proximate a second portion of the gas cell; a third
waveguide coupled, along a third dimension differing from the first
dimension, between the second end of the first waveguide and the
first portion of the gas cell; and a fourth waveguide coupled,
along a fourth dimension differing from the second dimension,
between the second end of the second waveguide and the second
portion of the gas cell.
2. The apparatus of claim 1 wherein the first dimension and the
second dimension are a same dimension.
3. The apparatus of claim 2 wherein the third dimension and the
fourth dimension are approximately perpendicular to the same
dimension.
4. The apparatus of claim 1 wherein the third waveguide and the
fourth waveguide comprise rectangular waveguides.
5. The apparatus of claim 1 wherein the third waveguide and the
fourth waveguide comprise metallic waveguides for communicating a
wave from the transceiver to the gas cell via an air medium.
6. The apparatus of claim 1 wherein the third waveguide and the
fourth waveguide comprise glue.
7. The apparatus of claim 1 wherein the third waveguide and the
fourth waveguide comprise polymer.
8. The apparatus of claim 1 wherein the third waveguide and the
fourth waveguide comprise solder balls, the waveguide formed by an
area surrounded by the solder balls.
9. The apparatus of claim 1 and further comprising apparatus for
retaining the gas cell in the second fixed position.
10. The apparatus of claim 9 wherein the apparatus for retaining
comprises a receptacle member comprising a cavity in which the gas
cell is positioned.
11. The apparatus of claim 10 wherein the receptacle member is
affixed to the substrate.
12. The apparatus of claim 11 and further comprising a cover
affixed adjacent at least a portion of the gas cell, the cover
further affixed relative to the receptacle member.
13. The apparatus of claim 9 wherein the apparatus for retaining
comprises a cover affixed adjacent at least a portion of the gas
cell, the cover further affixed relative to the substrate.
14. The apparatus of claim 9: wherein the apparatus for retaining
has a cavity for receiving the gas cell; and wherein the apparatus
for retaining comprises the third waveguide and the fourth
waveguide.
15. The apparatus of claim 14: wherein the third waveguide is
configured to couple a wave between the first waveguide and a first
electromagnetic wave passageway in the first portion of the gas
cell; and wherein the fourth waveguide is configured to couple a
wave between the second waveguide and a second electromagnetic wave
passageway in the second portion of the gas cell.
16. The apparatus of claim 1 wherein the gas cell comprises: a
first semiconductor wafer layer having a first cavity region; and a
second semiconductor wafer layer having a second cavity region.
17. The apparatus of claim 16 wherein each of the first cavity
region and the second cavity region comprises a trapezoidal cross
section.
18. The apparatus of claim 1 and further comprising a gas stored in
the gas cell.
19. The apparatus of claim 18 wherein the gas is selected from a
set consisting of HCN, DCN, OCS, H2O, and CH3CN.
20. A millimeter wave apparatus, comprising: a substrate; a
transceiver in a first fixed position relative to the substrate,
the transceiver configured to transmit and receive a millimeter
wave; at least one coupling from the transceiver to a wave signal
launch affixed to the substrate; and an interposer affixed to the
substrate and having an interposer waveguide aligned to the wave
signal launch, the interposer waveguide configured to communicate
the millimeter wave.
21. The millimeter wave apparatus of claim 20 wherein the at least
one coupling comprises a first coupling, the wave signal launch
comprises a first wave signal launch, and the interposer waveguide
comprises a first interposer waveguide, and further comprising: a
second coupling from the transceiver to a second wave signal launch
affixed to the substrate; and the interposer further comprising a
second interposer waveguide aligned to the second wave signal
launch, the second interposer waveguide configured to communicate
the millimeter wave.
22. The millimeter wave apparatus of claim 20 wherein the at least
one coupling comprises a first coupling, the wave signal launch
comprises a first wave signal launch, and the interposer waveguide
comprises a first interposer waveguide, and further comprising: a
plurality of couplings from the transceiver, in addition to the
first coupling, wherein each coupling in the plurality of couplings
is coupled to a respective wave signal launch affixed to the
substrate; and the interposer further comprising a plurality of
interposer waveguides, wherein each interposer waveguide in the
plurality of interposer waveguides is aligned to a respective one
of the respective wave signal launches, and wherein each of the
plurality of interposer waveguides is configured to communicate the
millimeter wave.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to, the benefit of the
filing date of, and hereby incorporates herein by reference: U.S.
Provisional Patent Application No. 62/669,598, entitled "COMPACT
MM-WAVE MOLECULAR CLOCK," filed May 10, 2018.
BACKGROUND
[0002] The example embodiments relate to a precision compact
millimeter wave systems (30 GHz to 300 GHz) systems, such as a
molecular clock operating based on rotational quantum response in
the clock cell.
[0003] Precision clock signals, usable as a base frequency source
either directly, or converted (e.g., divided down) to some multiple
of a base frequency source, can be generated from various circuits
and configurations. One precision clock signal example is an atomic
clock, so named as its signal is produced in response to the
natural and quantum response of atoms or molecules, to an
excitation source. In one approach, such atoms are in the form of
alkali metals stored in a chamber, where the excitation source can
be one or more lasers directed to the cell and the response of the
chamber atoms is detected by measuring the amount of laser energy
(photons) that passes through the chamber as the laser frequency
sweeps across a range. In another approach, such molecules are in
the form of dipolar gases also stored in a chamber, where the
excitation source is an electromagnetic wave propagating through
the chamber and the response of the chamber atoms is detected by
measuring the amount of electromagnetic energy that passes through
the chamber as the energy source sweeps across a range.
[0004] Further to the above, an example of a millimeter wave atomic
clock is described in U.S. Pat. No. 9,529,334 ("the '334 Patent"),
issued Dec. 27, 2016, hereby incorporated fully herein by
reference, and which is co-assigned to the same assignee as the
present application. The '334 Patent illustrates, among other
things, an atomic clock apparatus including a sealed cavity storing
a dipolar gas, with an electromagnetic entrance into which an
electromagnetic wave (or field) enters near a first end of the
cavity and an electromagnetic exit from which an electromagnetic
wave exits near a second end of the cavity. The electromagnetic
wave that so exits is measured to determine an amount of absorption
by (or transmission through) the dipolar gas, with the measure
indicative of the quantum response of the gas as a function of the
wave frequency.
[0005] Example embodiments are provided herein that build on
certain of the above concepts, as further detailed below.
SUMMARY
[0006] A millimeter wave apparatus, with a substrate, a transceiver
in a first fixed position relative to the substrate, and a gas cell
in a second fixed position relative to the substrate. The clock
apparatus also comprises at least four waveguides: (i) a first
waveguide affixed relative to the substrate, the first waveguide
having a first end coupled to the transceiver and a portion, along
a first dimension, having a second end proximate a first portion of
the gas cell; (ii) a second waveguide affixed relative to the
substrate, the second waveguide having a first end coupled to the
transceiver and a portion, along a second dimension, having a
second end proximate a second portion of the gas cell; (iii) a
third waveguide coupled, along a third dimension differing from the
first dimension, between the second end of the first waveguide and
the first portion of the gas cell; and (iv) a fourth waveguide
coupled, along a fourth dimension differing from the second
dimension, between the second end of the second waveguide and the
second portion of the gas cell.
[0007] Other aspects are also disclosed and claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1A is a plan view of portions of a compact molecular
clock system.
[0009] FIG. 1B illustrates the system of FIG. 1A, with the addition
of its transceiver coupled to an atomic clock cell assembly.
[0010] FIG. 1C illustrates a cross-sectional view of coplanar
waveguide of the system of FIG. 1A.
[0011] FIG. 2A is an exploded view of the gas cell assembly of FIG.
1.
[0012] FIG. 2B is a perspective view of portions of a wave path
through various example embodiment structure.
[0013] FIG. 3 is an exploded view of a portion of the gas cell of
FIG. 2A.
[0014] FIG. 4A is a plan view, and FIG. 4B is a cross-sectional
view, of an alternative compact molecular clock system.
[0015] FIG. 5A is a plan view, and FIG. 5B is a cross-sectional
view, of an alternative compact molecular clock system.
[0016] FIG. 5C is a cross-sectional view of an alternative compact
molecular clock system.
[0017] FIG. 6 is a cross-sectional view of an alternative compact
molecular clock system.
[0018] FIG. 7A is a plan view, and FIG. 7B is a cross-sectional
view, of an alternative compact molecular clock system.
[0019] FIG. 8A is a plan view, and FIG. 8B is a cross-sectional
view, of an alternative compact molecular clock system.
DETAILED DESCRIPTION
[0020] FIGS. 1A through 1C illustrate various views of an example
embodiment compact molecular clock system 100. Specifically, FIG.
1A illustrates a plan view of a substrate 102, such as a printed
circuit board (PCB), to which additional items are added to form
the molecular clock system. Substrate 102 is rectangular by way of
example with dimensions length (L) by width (W), where by way of
example L may be 3 to 4 inches, and W may be 2 to 3 inches.
Substrate 102 physically supports various electrical/electronic
elements and also facilitates electrical connectivity between
various of those elements. Thus, various blocks and electrical
traces are shown generically as examples. Substrate 102 also may
include many layers, such as in the case of a multilayer PCB
stackup.
[0021] FIG. 1B illustrates system 100 of FIG. 1A, which includes a
transceiver 104 coupled to an atomic clock cell assembly 106.
Transceiver 104 is, for example, an integrated circuit operable to
transmit and receive signals. As one example, transceiver 104 is an
integrated circuit radar, such as the AWR1642 which is commercially
available from Texas Instruments Incorporated. The AWR1642 is a
self-contained frequency-modulated continuous wave radar (FMCW)
sensor with radar sensors in the band of 76 to 81 GHz. The AWR1642
includes one or more processors (e.g., digital signal processor)
and supports multiple transmit and receive radar channels, radio
configuration, control, calibration, and programming of model
changes for enabling a wide variety of sensor implementations. FMCW
is just one approach as an example, and other embodiments are
contemplated with alternative transmission/modulation schemes
employed by transceiver 104. Accordingly, and as detailed below,
transceiver 104 is coupled to communicate with radar waveguides
between transceiver 104 and atomic clock cell assembly 106.
[0022] The radar waveguides between transceiver 104 and atomic
clock cell assembly 106 may be achieved, for example, by coplanar
waveguides shown generally at 108 and 110. One coplanar waveguide
108 may transmit a wave from transceiver 104, via a first antenna
112 (FIG. 1A), to cell assembly 106, and another coplanar waveguide
110 may receive a wave from cell assembly 106 and communicate it,
via a second antenna 114 (also, FIG. 1A), to transceiver 104. While
the current illustration provides a single transmit waveguide and a
single receive waveguide, plural of such waveguides are also
contemplated. Indeed, two transmit and two receive waveguides may
be used with axi-symmetric complementary devices so as to
accommodate PCB or other configuration constraints. In any event,
coplanar waveguide 108 (and 110) may, as an example, be formed,
plated, or etched on a surface or within a layer of substrate 102.
By way of example, FIG. 1C illustrates a partial cross-sectional
view of coplanar waveguide 108. Accordingly, the FIG. 1C
illustration provides context for the term "coplanar" as waveguide
108 includes a center conductor 108CC, formed by example as a
microstrip. Coplanar with center conductor 108CC is a ground plane
108GP, which is a metallic layer that is partially atop, along with
center conductor 108CC, a dielectric layer 108DL1, leaving gaps
108GA between the outer edges of center conductor 108CC and ground
plane 108GP. As known in the art, waveguide 108 is well-suited to
propagate, and maintain most of the electromagnetic wave energy,
along center conductor 108CC.
[0023] Waveguide 108 also may use conductive vias 108CV1 to extend
downward from ground plane 108GP, to connect to one or more common
grounded buried layers, as follows. A first grounded buried layer
108GBL1 is shown as the thin portion of conductive material between
dielectric layer 108DL1 and a second dielectric layer 108DL2,
thereby forming a grounded coplanar waveguide, which is often more
desirable for wave communication, albeit requiring the additional
structure to accomplish the common ground. A second grounded buried
layer 108GBL2 is spaced apart from layer 108GBL1 by a dielectric
layer 108DL3, which is thicker than dielectric layer 108DL2. As
further discussed below in connection with FIG. 2B, a portion of
dielectric layer 108DL3, and of second grounded buried layer
108GBL2, extends below first antenna 112 or second antenna 114
(FIG. 1A), whereby those portions provide a reflector to guide
waves through the respective antennas and confine the energy from
dissipating laterally. Lastly and returning to FIGS. 1A and 1B,
note that as waveguides 108 and 110 approach assembly 106, each
turns in a direction that is approximately orthogonal relative to a
respective flat edge 106E of assembly 106, whereupon each waveguide
108 and 110 may enter into a respective recess formed on the
backside of an interposer (see, a recess 204R on backside of an
interposer 204 in FIG. 2A), where each recess is formed from a
respective edge 106E to a respective one of first antenna 112 or
second antenna 114 on substrate 102, which desirably reduces
cross-talk.
[0024] FIG. 2A is an exploded view of assembly 106, relative to a
portion of substrate 102, and referring thereto atomic clock cell
assembly 106 is now described. In an example embodiment, assembly
106 includes four items, namely, a back plate 202, an interposer
204, an atomic gas cell 206, and a top plate 208, all of which are
positioned relative to substrate 102. Each of these items is
further described below.
[0025] Back plate 202 may be metal, and it is positioned on one
side (e.g., bottom) of substrate 102. For fitment of back plate 202
relative to substrate 102, back plate 202 includes one or more
alignment pins 210 that fit through respective apertures 212 in
substrate 102. For reasons described below, back plate 202 also
includes four (e.g., threaded) apertures 214, one such aperture
near each of its corners. When back plate 202 is affixed to
substrate 102, each of the apertures 214 aligns with a respective
aperture 216 through substrate 102.
[0026] Interposer 204 may be made of metal, such as copper,
aluminum, or made of plastic or polymer and subsequently plated
with copper, silver or gold, and preferably is of high
conductivity, whereas by contrast back plate 202 and top plate 208
do not necessarily comprise high conductivity materials. Interposer
204 is positioned on a side (e.g., top) of substrate 102 opposite
the side at which back plate 202 is positioned. In an example
embodiment, the alignment pins 210 of the back plate 202 extend not
only into apertures 212 in substrate 102, but also through
substrate 102 so that the tips of those pins 210 fit within
respective apertures on the surface of interposer 204 that contacts
the substrate 102 (but that surface is not visible in the
perspective of FIG. 2A). Further, once interposer 204 is positioned
against substrate 102, four fasteners (e.g., screws, not shown) are
affixed through four respective countersunk apertures 218 in
interposer 204, through respective apertures 216 in substrate 102,
and affix (e.g., threadably, pressure fit) within respective
apertures 214 in backplate 202. Accordingly, these fasteners ensure
a retaining compressive force, and an established or enforced
alignment of interposer 204 relative to substrate 102, by affixing
back plate 202 and interposer 204, with substrate 102 compressed
therebetween. Further with respect to interposer 204, it also
includes a cavity 220, for example generally parallelepiped in
shape, and as detailed below to receive gas cell 206 to enforce
alignment between the waveguide and the physics cell launches. A
first rectangular aperture 222 is proximate a first end of cavity
220, and a second rectangular aperture 224 is proximate a second
end of cavity 220, where each of rectangular apertures 222 and 224
extend from cavity 220 through the remainder of the metal material
of interposer 204. In this regard, when interposer 204 is affixed
to substrate 102, a portion of coplanar waveguide 108 becomes
located between interposer 204 and substrate 102 and that portion
aligns within a recess 204R along the backside of interposer 204.
Further, rectangular aperture 222 of interposer 204 aligns with
first antenna 112, which is connected to waveguide 108. Similarly
when interposer 204 is affixed to substrate 102, a portion of
coplanar waveguide 110 becomes located between interposer 204 and
substrate 102 and that portion aligns within another recess (not
shown) along the backside of interposer 204, relative to
rectangular aperture 224. Further, rectangular aperture 224 aligns
with second antenna 114, which is connected to waveguide 110.
[0027] First and second antennas 112 and 114 were introduced
earlier, but as may be now better appreciated, are formed, for
example, as metal couplers with an outside shape to generally match
that of rectangular apertures 222 and 224. In the illustrated
example, therefore, for each antenna 112 and 114, an outer
rectangular metal shape is provided with a center opening, such as
concentrically located within each rectangle. These antennas 112
and 114 or coupling structures are designed to maintain a
continuous geometrical structure and thereby minimize both
impedance mismatch and the insertion loss to the aperture 222 and
224 and in this way efficiently conduct the signal to and from gas
cell 206. The dimensions (and shaping) may be designed so as to
communicate wave signals as further described later, for example
according to the frequency band of the signals. In the examples
provided, the shaping provides "rectangular waveguide" (RW)
structures for guiding electromagnetic waves although not
necessarily limited thereto, where such structures are also
sometimes referred to as transmission lines. Lastly and for reasons
described below, interposer 204 also includes four threaded
apertures 226 on its surface that faces away from substrate 102,
where preferably each of these threaded apertures 226 does not
extend fully through the thickness of interposer 204.
[0028] Cell 206 includes a sealed interior in which a gas is
stored. More specifically, cell 206 stores a dipolar gas, such as
water vapor or any other dipolar molecular gas, inside an enclosed
cavity of the cell, the cavity being sealed by nature of shapes,
layering, and the like that combine to enclose the dipolar gas at a
relatively low (e.g., 0.1 mbar) pressure. A particular dipolar gas
is preferably selected based on a frequency range of interest. For
example, with transceiver 104 providing a frequency range of 76 to
81 GHz, an appropriate dipolar gas may be HCN, DCN, OCS, H2O, CH3CN
etc. Additionally, cell 206 may be formed in connection with an
integrated circuit wafer, which can include multiple layers affixed
relative to a semiconductor substrate (see, e.g., the incorporated
by reference U.S. Pat. No. 9,529,334). Preferably, the outer
perimeter of cell 206 is shaped to fit in abutment with the inner
walls/shape of cavity 220 of interposer 204. Accordingly, once
interposer 204 (and back plate 202) is fixed relative to substrate
102, cavity 220 in essence provides a receptacle in which cell 206
may be located, and later removed/replaced if desired or necessary.
Further with respect to cell 206 and such alignment, the bottom
surface 228 of cell 206, that aligns with the bottom of cavity 220,
is not visible from the perspective of FIG. 2A. As shown in dotted
lines, however, that bottom surface 228 includes a first
rectangular antenna transition 230 that, when cell 206 is within
cavity 220, aligns and is in wave communication with, aperture 222,
so that antenna transition 230 functions as a wave launch;
similarly, bottom surface 228 includes a second rectangular antenna
transition 232 that, when cell 206 is within cavity 220, aligns and
is in wave communication with, aperture 224, so that antenna
transition 232 also functions as a wave launch. A waveguide 234 is
formed between transitions 230 and 232, which may be formed by the
shape and/or material of the cell cavity, further enhanced by the
inclusion of metal along one or more of the surfaces of the cell
cavity, an example of which is described below.
[0029] Having illustrated and described a wave communication path
between substrate 102 and cell 206, through interposer 204, FIG. 2B
illustrates a perspective view of portions of that path. In FIG.
2B, certain reference numbers introduced earlier are repeated.
Accordingly, interposer 204 is adjacent substrate 102, and a
portion of coplanar waveguide 108 is show on substrate 102 and
passes into recess 204R of interposer 204. Along the sides of
coplanar waveguide 108 are positioned a number of approximately
evenly-spaced conductive vias 108CV1. Coplanar waveguide 108 ends
at antenna 112, which is shown aligned with rectangular aperture
222 of interposer 204. While not shown, beneath antenna 112 there
is an opening, for example rectangular, through grounded buried
layer 108GBL1 (FIG. 1C) so that reflections from second grounded
buried layer 108GBL2 (FIG. 1C) may reach antenna 112, through
dielectric layer 108DL3 (FIG. 1C). Around the outer perimeter of
antenna 112 are conducive vias 108CV2, which are longer than
conductive vias 108CV1, as conducive vias 108VC2 extended deeper
into the layers of substrate 102 so as to reach grounded buried
layer 108GBL2 (FIG. 1C). In this way, that grounded buried layer
108GBL2, and dielectric layer 108DL3 (FIG. 1C) above it, facilitate
the above-described reflector functionality with respect to a wave
traveling along coplanar waveguide 108 and within aperture 222.
[0030] FIG. 3 is an exploded partial view of an example
construction of one end of cell 206, with it understood that the
other end is comparably constructed. Cell 206 is preferably formed
from two like-dimensioned (e.g., same thickness) semiconductor
wafer layers 302 and 304. Each wafer layer 302 and 304 may be
etched at a same time so as to achieve uniform etch shape and
dimensions. In an example embodiment, the wafer layer etch can be
achieved using tetramethylammonium hydroxide (TMAH) etching and to
form respective trapezoidal cross-section cell cavity regions 306
and 308 along a respective length of layers 302 and 304. Such a
shape may be desirable for interfacing with the TI AWR 1642 in
E-band. Other cross-sections are also contemplated, including as
examples KOH (Potassium Hydroxide), DRIE (Deep Reactive Ion
Etching), RIE (Reactive Ion Etching), XeF2 (Xenon difluoride), etc.
Also, the etch may be constrained in contemporary processes for
smaller dimension etching, and when forming an angle along the etch
may, by virtue of the etch process and not necessarily otherwise by
specification, fix the trapezoid leg angle at approximately 54.7
degrees. In all events, etch attributes may be selected based on
the absorption frequency of the molecule in cell 206. The width and
depth of the etched cavity will define the cross-section of the
"metallic waveguide" formed when all sides of the cavities are
metallized later on in the process flow. The dimensions of the
metallic waveguide will define a cut-off frequency below which
there will be no electromagnetic waveguide propagation, as well as
an upper cut-off frequency, which are common knowledge. Then, the
dimensions of the cavity should be designed such that the quantum
transition to be interrogated in the cell is at least 10 GHz above
this cut-off frequency to ensure a good electromagnetic signal
propagation. For example, a transceiver/interposer/cell at 73 GHz
is roughly twice the size of the same set of structures to
interrogate the same molecule at 182 GHz. Examples of other such
structures may be found in co-owned U.S. Pat. Nos. 9,529,334 and
10,131,115, and co-owned U.S. patent application publications
US2019/0074233 and US2019/0071306A1, all of which are hereby
incorporated fully herein by reference. In the present example,
using such an etch in a single wafer would cause the bottom of the
etch (the base width of the trapezoid at the depth of the etch) to
be considerably shorter than the top of the etch (the base width of
the etch at the upper surface of the wafer). However, in an example
embodiment, by etching two wafers 302 and 304 and then combining
them as described below, the etch limitations only apply to each
wafer, at a shorter total depth. In other words, by way of example,
each wafer 302 and 304 is etched to a depth of 0.7 mm, with a
resulting bottom base edge approximately 2 mm long and a top base
edge approximately 3.1 mm; when wafers 302 and 304 are therefore
subsequently faced to one another as shown in FIG. 3, the resultant
cavity 309 comprising cavity regions 306 and 308 will have a
hexagonal cross-sectional shape with a total height of
approximately 1.4 mm; to the contrary, were a single trapezoid etch
performed in a single wafer to form the cavity, the fixed trapezoid
leg angle of the TMAH etch would result in a shorter bottom (e.g.,
1 mm or less), which may be less desirable as the cavity
subsequently performs its waveguide functionality. While cavity 309
is shown by example as hexagonal, alternative embodiments are
contemplated in which cavity 309 has a cross-section that provides
a circular waveguide and/or a circular polarized propagation
mode.
[0031] Additional aspects with respect to the layering of cell 206
are also shown in FIG. 3, as now described. With respect to layer
302, in addition to the above-described trapezoidal etch, an
aperture 310 is formed proximate one end of cavity region 306 and
from cavity region 306 to a surface 312 of layer 302. Aperture 310,
therefore, forms an electromagnetic wave passageway for one of the
two above-described rectangular antenna transitions 230 and 232,
while similarly, but not shown in the partial view of FIG. 3, a
comparable second aperture is formed proximate an opposite end of
cavity region 306, so as to form an electromagnetic wave passageway
for the other of the two above-described rectangular antenna
transitions 230 and 232. Surface 312 is also metalized to form a
metalized layer 314, as may be achieved using known processes.
Thereafter, cavity 306 is positioned to face cavity 308, with a
bonding ring 316 between layers 302 and 304 and preferably
positioned just beyond the outer boundary of cavity regions 306 and
308. Bonding ring 316 may be a eutectic metal, deposited for
example by sputtering, e-beam evaporation, or electroplating, and
used to assist with bonding wafers 302 and 304 together, thereby
creating a singular cavity 309 from cavity regions 306 and 308.
Also, while not shown, either or both of cavity regions 306 and 308
may be treated, lined, coated, or otherwise processed to include
additional aspects, such as conductive or dielectric layering, to
facilitate or improve the interior of cavity 309 as a signal
waveguide and to minimize surface reactivity to the gas contained
in the cavity or to prevent outgassing from the sidewalls into the
cavity. Thus, the waveguide will communicate a wave along the
created, singular cavity 309, from aperture 310 at one end of the
cavity to the other aperture at the other end of the cavity (not
shown, but represented in FIG. 2A as associated with one of
transitions 230 and 232).
[0032] Completing the illustration of FIG. 3, cell 206 also may
include a glass sheet 318 and a metal coupling ring 320. Glass
sheet 318 has generally the same outer dimensions as layers 302 and
304, and may be 200 to 300 microns thick, by way of example. Glass
sheet 318 provides a plane on which can be patterned antennas on
its top surface so as to permit the launch of the wave signal into
cavity 309. Selection of glass as a material for this plane (for
glass sheet 318) may be desirable as glass provides a dielectric
constant of around 4 to 5, permitting the stated thickness of
200-300 microns. To the contrary, for example, if silicon were
used, it would provide a dielectric constant of 13, implying a
layer thickness well below 100 microns, which will make the
mechanical construction much more complicated. Indeed, such a thin
membrane may fail to hold the pressure differential between the
inside and the outside of the cell. Other materials that are
matched, in terms of coefficient of thermal expansion, with silicon
and that provide low dielectric constant are also candidates for
layer 318. Metal coupling ring 320 is affixed to glass sheet 318,
at a position to align with aperture 310, and is described as a
ring in that it presents a metal structure with a central aperture
322, where again both the surrounding metal and the aperture 322
are rectangular, consistent with the other rectangular waveguides
herein.
[0033] Returning to FIG. 2A and having detailed cell 206 in
connection with FIG. 3, the placement of the glass sheet 318 side
of cell 206 into cavity 220 of interposer 204 completes a wave path
between antennas 112 and 114. By way of example, therefore,
transceiver 104 may transmit a wave along waveguide 108, along a
first dimension parallel to substrate 102, to antenna 112 as a
transmit antenna. From antenna 112, the wave will continue, but in
a second dimension differing from the first dimension (i.e., not
parallel to substrate 102), where in the example of FIG. 2A this
second dimension is perpendicular (or approximately perpendicular,
such as 90.+-.10 degrees) to the first dimension. In the
illustrated example, the second dimension guides the wave through
the air medium inside first rectangular aperture 222 of interposer
204, as a rectangular waveguide, further through central aperture
322 of metal coupling ring 320 (FIG. 3), glass sheet 318, and
aperture 310 proximate one end of cavity region 306, all serving as
first rectangular antenna transition 230 and thereby entering the
resultant cavity 309 of cell 206. Once the wave travels along that
resultant cavity 309 of cell 206, it will interrogate atoms of the
dipolar gas inside the cell will respond based on the frequency of
the interrogating wave. Thus, the wave will continue along cavity
309 and then exit from a comparable central aperture (not shown),
pass again in the second dimension through glass sheet 318, and
then another metal coupling ring (not shown), all serving as second
rectangular antenna transition 232, from where the wave will
continue through the air medium inside second rectangular aperture
224 of interposer 204, also as a rectangular waveguide, to antenna
114. Once the wave reaches antenna 114, it may be communicated, in
the first dimension, by paired trances 110 to transceiver 104. As a
result, transceiver 104 may evaluate the received signal response
and, for example in comparison to the energy of the transmitted
wave signal, make various determinations, such as whether (or when)
the frequency of the excitation wave matches the rotational quantum
transition frequency of the dipole gas in the cell.
[0034] Completing FIG. 2A, top plate 208 is secured atop a portion
or all of cell 206 by affixing to plate 208 to interposer 204. More
specifically, once top plate 208 is positioned against atop cell
206 and adjacent interposer 204, four fasteners (e.g., screws, not
shown) are affixed through four respective countersunk apertures
236 in top plate 208 and affix (e.g., threadably) within respective
apertures 226 in interposer 204. Accordingly, these fasteners
ensure a compressive force, and established alignment and
retention, between cell 206 and interposer 204.
[0035] From the preceding, system 100 provides a compact millimeter
wave system in which an interposer provides a wave path directly to
a PCB launch, where the interposer includes another waveguide, such
as a standard WR structure, for example a WR-12 flange. As a
result, a gas cell can be easily and quickly tested by locating it
within the interposer using standard lab instrumentation, without
the need for wafer probing directly to the PCB launch. This is a
potentially considerable benefit, as the alternative of wafer
probe, especially at millimeter wave geometry, is costly, tedious,
and has significant repeatability challenges. Conversely, example
embodiments facilitate affixing (e.g., screwing) pieces together
with much less risk to both the part and the instrumentation. And,
an assembled physics cell/interposer can be quickly tested without
critical microscope optical alignment and then be then readily
assembled to the millimeter wave transceiver PCB. Still further, in
the example millimeter wave clock example described, it may use an
already-existing transceiver 104 (e.g., TI AWR 1642), located on a
substrate 102 (e.g., PCB) away from the clock gas cell 206. The
system 100 further includes: (i) a transmit and receive waveguide
108, 110 affixed to (e.g., atop; within) the substrate 102,
extending between the transceiver 104 and ends of the gas cell 206;
and (ii) two additional waveguides 222, 224, each extending away
from a respective one of the paired waveguides, and in a dimension
other than that of the paired waveguide (e.g., perpendicular), to
an end of the gas cell. Numerous other aspects also have been shown
in connection with system 100. For example, from the wave path just
described, the wave may enter the cell, travel through it, and
again return, via the second dimension, to the dimension of another
pair of waveguides and back to the transceiver. As another example,
the cell may be positioned in a receptacle, where the receptacle is
rigidly affixed relative to antennas on a substrate so as to reduce
potential signal loss as the wave propagates, particularly as it
passes through media of different impedance. Moreover, the
inventive scope includes various other example embodiments, which
may be separately considered within the present scope and from
which selected features of different embodiments may be combined to
form still other example embodiments, as will be understood by one
skilled in the art from the remaining discussion.
[0036] Further from the preceding, various of the preceding
inventive teachings, as well as other that follow, may be applied
to other millimeter wave systems. Specifically, typical testing may
be achieved by either mechanical, hand, and/or machine-guided or
implemented probing. Often in this context, a substrate (e.g., PCB)
has printed guidelines on the substrate akin to crosshairs, and
test probe alignment may be aligned to the guidelines while
attempting to use a (sometimes crude) microscope, with sufficient
magnification (e.g., 250 times) so that the probe tip may be
observed while bringing it into contact with a proper landing
point. Sometimes multiple probes are so moved at once, requiring
proper and concurrent guidance of each probe to a respective
landing point and a same time, with little room for error. And, the
efforts must be repeated for both the transmit and receive
millimeter wave paths. Unsurprisingly, such an approach is very
time intensive and prone to error. In contrast, example embodiments
provide an interposer affixed to the millimeter wave communicating
substrate and having waveguides corresponding to target locations
on the substrate, whereby the interposer is thusly aligned with
respect to those target locations that otherwise would, in the
prior art, require the above-noted probing. Accordingly, the
interposer provides a testing mechanism already aligned relative to
the millimeter wave path item to be tested. In the examples
provided herein, therefore, interposer 204 provides an affixed
cavity 220 already aligned to millimeter wave communication points
represented by first antenna 112 and second antenna 114. Hence,
further testing, or millimeter wave communication, need not be
directly to those communication points, but instead may be made via
the interposer. In the example of a millimeter wave clock,
therefore, interposer 204 serves as an already-aligned receptacle
in which atomic gas cell 206 may be located with far less
complexity and time than would be required to precisely align it
with, and affix it directly to, first antenna 112 and second
antenna 114.
[0037] FIG. 4A is a plan view, and FIG. 4B is a cross-sectional
view, of an alternative compact molecular clock system 400. System
400 includes a substrate 402 (e.g., PCB or the like) and a
transceiver 404 affixed to substrate 402. Transceiver 404 can
electrically communicate with other apparatus fixed relative to
substrate 402. Such communications include between transceiver 404
and an atomic clock cell 406, by coplanar waveguides shown
generally at 408 and 410. In the illustrated embodiment, waveguides
408 and 410 may be formed by etching appropriate paths, preferably
in a same plane, from a metallic layer 411 that is located on or in
substrate 402. Waveguide 408 may communicate with a first antenna
area 412 and waveguide 410 may communicate with a second antenna
area 414.
[0038] Certain aspects of system 400 differ from system 100 of
FIGS. 1A through 3. For example, cell 406 again includes two
semiconductor wafers 416 and 418, each with a respective
trapezoidal cavity, faced and affixed to one another to form a
continuous resultant cavity 420, where in this example embodiment
cavity 420 has a partial serpentine path that perpendicularly
changes direction twice as the wave propagates between first
antenna area 412 and second antenna area 414. Additionally, cell
406 includes both an upper glass layer 422 and a lower glass layer
424, and cell 406 is not enclosed in a receptacle apparatus.
Instead, cell 406 is electrically (and physically) connected and
coupled to substrate 402 by a group of, preferably symmetrically
located, conductive affixation members 426, which in the
illustrated example embodiment are solder balls. In the example of
FIGS. 4A and 4B, the group of conductive affixation members also
may be arranged generally in row/column orientation, thereby
forming a ball grid array (BGA) or copper studs or bumps between
lower glass layer 424 and a surface of substrate 402. Conductive
affixation members 426 (e.g., solder balls), however, are not
present in the first antenna area 412 and second antenna area 414
(or in the path of waveguides 408 and 410). As a result, the
absence of a conductor in those areas, and the surrounding
conductors around the perimeter of those areas as best seen in FIG.
4A, form a metallic waveguide from the antenna areas 412 and 414,
vertically in FIG. 4B, upward through lower glass layer 424.
Accordingly, for example, a wave may travel from a transmit channel
of transceiver 404, through waveguide 408 to first antenna area
412, upward in air and through lower glass layer 424 and into cell
406 and through its serpentine path, while then exiting cell 406
again through lower glass layer 424, through air to second antenna
area 414, and then through waveguide 410 to a receive channel of
transceiver 404. Lastly, in this example embodiment (and others),
if the various waveguide structure path is dimensioned at least one
order smaller than the wavelength of the wave being guided, then
the wave effectively "sees" the communication path as a continuous
conductor, that is, with relatively small signal loss along the
wave path. Accordingly, in various example embodiments, the wave
path structure is such that at least in the vicinity of antenna
areas 412 and 414, the spacing of conductive affixation members 426
provide a passageway that is the wave wavelength/10, or smaller.
However, over the remainder of substrate 402, spacing of affixation
members 426 may vary or be determined by the mechanical design
considerations for thermal stress.
[0039] FIG. 5A is a plan view, and FIG. 5B is a cross-sectional
view, of an alternative compact molecular clock system 500. System
500 includes various of the same members and connectivity of system
400 in FIGS. 4A and 4B, so for such items like reference numbers
are carried forward from system 400 to system 500. For system 500,
however, a layer of glue 502 (with favorable dielectric constant
and loss tangent) is used to affix cell 406 relative to substrate
402, as opposed to conductive affixation members. Further, a
metalized layer 504 is formed outside of lower glass layer 424, as
may be achieved as a metalized layer akin to layer 314 of FIG. 3;
here, however, openings (e.g., by evaporation and patterning) are
made in metalized layer 504 to allow wave passage through the
openings so as to create a first antenna area 506 and a second
antenna area 508 and to create an electronic band gap structure
(EBG) for wave communication, again consistent with earlier
teachings. Thus, a wave may pass from one waveguide 408 in the area
of first antenna area 412, through the medium of glue 502 and lower
glass layer 424 and entering into cell 406, as guided by a first
opening in metalized layer 504. The wave then continues through
cell 406, and it then exits from an opposite end of cell 406 by
passing through lower glass layer 424 and the medium of glue 502,
as guided by a second opening in metalized layer 504 in the area of
second antenna 508, continuing then to waveguide 410. Further, in
an example embodiment, the thickness of glue 502 is sufficiently
thin so as to prevent metalized layer 504 and metallic layer 411
from acting as a parallel plate waveguide in order to prevent
unwanted cross-talk or signal loss
[0040] FIG. 5C illustrates an alternative cross-sectional view to
compact molecular clock system 500 of FIG. 5 and, accordingly, in
FIG. 5C the system is shown as system 510. Again, where comparable
items exist in FIG. 5C from an earlier Figure(s), the same
reference number(s) is carried forward. System 510 replaces the
glue layer 502 of FIG. 5B with sections 514 and 516 of high
dielectric constant polymer. In system 510, therefore, the wave
path is through the high dielectric constant polymer sections 514
and 516 rather than glue (e.g., FIG. 5B) or air (e.g., FIG. 4B), as
may be more favorable for certain implementations. Indeed, by
having a high dielectric constant polymer sections 514 and 516,
just in the antenna area, the transfer of energy between the
antenna to the gas cell is increased or maximized. In other words,
insertion loss can be substantially improved because the
electromagnetic waves would prefer to concentrate in the high
dielectric regions and minimize the propagation between TX and RX,
reducing cross talk. While cross talk may be mitigated with other
techniques (e.g., EBG for narrow band between TX and RX), the use
of low loss, high dielectric constant polymer placed in the area of
the antennas greatly assists against a parallel plate
electromagnetic mode that otherwise could be excited and increase
TX/RX cross talk. Lastly, the thickness of polymer sections 514 and
516 are selected according to the wavelength of the guided wave,
where preferably that thickness is less than or equal to
wavelength/4.
[0041] FIG. 6 illustrates a cross-sectional view of another
alternative compact molecular clock system 600. Again, where
comparable items exist in FIG. 5C from an earlier Figure(s), the
same reference number(s) is carried forward. In system 600, a
receptacle area 602 is formed as a cavity in substrate 402, thereby
forming a receptacle into which cell 406 is positioned. Waveguides
604 and 606 (606 shown with dashed lines, as not visible from the
cross section where 604 is visible) extend first vertically and
then horizontally (horizontally either co-planar, or not
necessarily) between transceiver 404 and receptacle area 602,
providing a first antenna area 608 and a second antenna area 610.
Accordingly, a wave may be communicated between cell 406 and a
respective one of the antenna areas 608 and 610, by passing through
a respective aperture in metalized layer 504 as well as lower glass
layer 424. For structural retention (and potentially alignment), a
cover 612 is also affixed atop either a portion or all of cell 406,
as may be further retained by affixation members 614. Indeed, with
the retention of cell 406 in this manner, further underlying
alignment support is unnecessary and, therefore, the horizontal
extension of waveguides 604 and 606 may directly contact metalized
layer 504 (that is formed on the exterior surface of lower glass
layer 424).
[0042] FIG. 7A is a plan view, and FIG. 7B is a cross-sectional
view, of another alternative compact molecular clock system 700.
Again, where comparable items exist in FIGS. 7A and 7B from an
earlier Figure(s), the same reference number(s) is carried forward.
In system 700, comparable in various respects to system 500 of
FIGS. 5A and 5B, cell 406 is positioned above substrate 402. For
system 700, however, paired covers 702 and 704 are included atop
cell 406, as may be further retained by affixation members 706.
Similar to system 600, in system 700 with the physical retention
and alignment of cell 406 by a cover or covers 702 and 704, further
underlying alignment support is unnecessary and, therefore, the
horizontal extension of waveguides 408 and 410 may directly contact
metalized layer 504.
[0043] FIG. 8A is a plan view, and FIG. 8B is a cross-sectional
view, of another alternative compact molecular clock system 800.
Again, where comparable items exist in FIGS. 8A and 8B from an
earlier Figure(s), the same reference number(s) is carried forward.
In system 800, comparable in various respects to system 500 of
FIGS. 5A and 5B, cell 406 is positioned above substrate 402. In
system 800, however, such positioning is achieved by a land grid
array (LGA) connection, which includes a number of solder pads 802
between substrate 402 and glass layer 424. As with FIGS. 4A and 4B,
the LGA connections in the vicinity of areas 412 and 414 are spaced
to provide a passageway that is the wave wavelength/10, or smaller.
Each pad may include a solder paste that is positioned between
metal layers, where a first of those metal layers can be the same
metallic layer 411 in which the TX/RX waveguides are formed, and a
second of those layers can be from portions of the metalized layer
504. The solder paste is subsequently melted, for example during a
reflow process, so as to align and affix cell 406 relative to first
antenna area 412 and second antenna area 414.
[0044] From the above, one skilled in the art should appreciate
that numerous example embodiments are provided, each representing a
compact molecular clock system. Example embodiment may have various
benefits. For example, example embodiments provide appropriate
alignment of the atomic gas cell relative to communication
antennas, for example to propagate waves through different media,
including air, glue, and polymer, while still other propagation
media may be included and/or substituted. As another example, some
example embodiments permit removal and replacement of the clock
cell, while also facilitating alignment within a receptacle,
including for example by human hands. As yet another example,
alignment is achieved in various manners for differing example
embodiments, so as to reduce or minimize signal loss. As a final
example, additional modifications are possible in the described
embodiments, and other embodiments are possible, within the scope
of the claims.
* * * * *